[clang] [llvm] [IR] Allow fast math flags on fptrunc and fpext (PR #115894)

[John Brawn via cfe-commits]

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@@ -1875,13 +1873,17 @@ Instruction *InstCombinerImpl::visitFPTrunc(FPTruncInst &FPT) {
       // fptrunc (select Cond, (fpext X), Y --> select Cond, X, (fptrunc Y)
       Value *NarrowY = Builder.CreateFPTrunc(Y, Ty);
       Value *Sel = Builder.CreateSelect(Cond, X, NarrowY, "narrow.sel", Op);
+      if (auto *I = dyn_cast<Instruction>(Sel))
+        I->setFastMathFlags(Op->getFastMathFlags());
       return replaceInstUsesWith(FPT, Sel);
     }
     if (match(Op, m_Select(m_Value(Cond), m_Value(Y), m_FPExt(m_Value(X)))) &&
         X->getType() == Ty) {
       // fptrunc (select Cond, Y, (fpext X) --> select Cond, (fptrunc Y), X
       Value *NarrowY = Builder.CreateFPTrunc(Y, Ty);
       Value *Sel = Builder.CreateSelect(Cond, NarrowY, X, "narrow.sel", Op);
+      if (auto *I = dyn_cast<Instruction>(Sel))
+        I->setFastMathFlags(Op->getFastMathFlags());
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john-brawn-arm wrote:

I don't think this is possible. Applying this patch first then later applying a patch to instcombine would cause test failures, as I mention in my comment above. Applying a patch to instcombine first doesn't work, because Instruction::getFastMathFlags fails an assert when the instruction isn't an FPMathOperator, which fptrunc wouldn't be at that point.

https://github.com/llvm/llvm-project/pull/115894